Pump piston assembly with acoustic dampening device

ABSTRACT

A pump piston assembly has a pump cylinder, at least one hollow piston slidably mounted for reciprocating motion in the pump cylinder, the piston having an inner surface of a first diameter and an outer surface of a second diameter, and a liner sleeve of a different material from the piston mounted on the inner or outer surface of the piston with a close or friction fit to the diameter of the piston surface on which it is mounted. The liner sleeve is of an acoustic dampening material which has a lower acoustic frequency than the piston material and is configured to absorb harmonic acoustic vibrations resulting from reciprocation of the piston in the cylinder.

BACKGROUND

1. Field of the Invention

The invention relates generally to piston assemblies for gas pumps and the like having at least one piston reciprocating in a cylinder, and is particularly concerned with an acoustic dampening device for reducing any squeaking noise generated as the piston slides in the cylinder.

2. Related Art

Gas piston pumps are used in medical ventilators and other applications. In some ventilator pumps, a piston of a first material such as graphite reciprocates in a cylinder of a second material such as glass. This has been found to generate squeaking noise between the graphite and glass due to acoustic resonance resulting from any slight misalignment between the piston and cylinder. This noise can occur even for tiny misalignments between the piston guide pin and the pump chamber or cylinder axis, and it is difficult to correct such tiny misalignments due to the number of components involved in such pump assemblies. Such squeaking noises are a problem to users of home ventilators and the like, particularly under high humidity conditions when the squeaking is more prevalent.

Therefore, what is needed is a system and method that overcomes these significant problems found in the conventional systems as described above.

SUMMARY

It is an object of this invention to provide a pump piston assembly with an acoustic dampening device which reduces or eliminates the squeaking noise due to acoustic resonance when the piston reciprocates in the chamber or cylinder.

In one aspect, a piston assembly is provided, which comprises a cylinder of a first material, at least one piston of a second material different from the first material slidably mounted for reciprocating motion in the cylinder, the piston being hollow and having an inner surface of a first diameter and an outer cylindrical surface of a second diameter, and at least one liner sleeve mounted on the inner or outer surface of the piston and having a close tolerance fit or interference fit to the piston surface on which it is mounted so that there is wall to wall contact between the opposing surfaces of the liner sleeve and piston. The liner sleeve is of an acoustic dampening material which has a lower acoustic frequency than the piston material and is configured to absorb harmonic acoustic vibrations resulting from reciprocation of the piston in the cylinder.

The liner sleeve acts as an acoustic dampening device so as to absorb or reduce any squeaking noises as the piston reciprocates in the cylinder, and may be mounted on the outer surface of the piston or inside the hollow piston. The sleeve is configured such that there is close wall to wall contact between the piston and liner sleeve. The sleeve may be of any suitable acoustic dampening material of relatively low acoustic frequency, and may be of hard plastic material, rubber, or the like. The sleeve may be attached to the inner or outer surface of the piston by an adhesive, or may be press fit inside or outside the piston.

Other features and advantages of the present invention will become more readily apparent to those of ordinary skill in the art after reviewing the following detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The details of the present invention, both as to its structure and operation, may be gleaned in part by study of the accompanying drawings, in which like reference numerals refer to like parts, and in which:

FIG. 1 is a simplified longitudinal cross-sectional view of one embodiment of a piston assembly for a gas pump incorporating an acoustic dampening device;

FIG. 2 is a longitudinal cross-sectional view similar to FIG. 1 but illustrating an alternative configuration of the acoustic dampening device;

FIG. 3 is a perspective view of another embodiment of an acoustic dampening device for use in a piston assembly of a gas pump or the like;

FIG. 4 is a longitudinal cross-sectional view similar to FIGS. 1 and 2 illustrating a second embodiment of a piston assembly incorporating the acoustic dampening device of FIGS. 3; and

FIG. 5 is a simplified perspective view of a gas pump assembly for a medical ventilator, with an acoustic dampening device as in FIG. 1, 3 or 4 associated with each of the pistons.

DETAILED DESCRIPTION

Certain embodiments as disclosed herein provide for a gas pump assembly of a medical ventilator or the like incorporating an acoustic dampening device to reduce or eliminate the squeaking noise of one or more pistons of the pump assembly sliding in its respective pump chamber or cylinder.

After reading this description it will become apparent to one skilled in the art how to implement the invention in various alternative embodiments and alternative applications. However, although various embodiments of the present invention will be described herein, it is understood that these embodiments are presented by way of example only, and not limitation. As such, this detailed description of various alternative embodiments should not be construed to limit the scope or breadth of the present invention as set forth in the appended claims.

FIG. 1 illustrates a first embodiment of a piston assembly 10 for a gas pump or the like incorporating an acoustic dampening device or liner sleeve 12. Piston assembly 10 basically comprises a hollow piston 14 slidably contained within a cylinder 15 inside pump housing 16. A piston rod 18 extends from an end wall 19 of the piston through a diaphragm seal 20 in one end of the cylinder and is linked via a sliding drive linkage 34 to a drive assembly or unit which drives piston 14 to reciprocate back and forth along drive axis 22 in cylinder 15. The linkage 34 illustrated in FIG. 1 is configured to convert rotational movement of a drive shaft into axial movement of piston 14 and piston rod 18, as described in more detail in U.S. Pat. No. 7 654 802 of Crawford, Jr. et. al., the contents of which are incorporated herein by reference. However, the acoustic dampening device or liner sleeve 12 may be installed on any piston where noise generated by sliding of the piston in its associated cylinder is a problem, regardless of the piston drive mechanism, and any suitable drive assembly and drive linkage may be used to drive piston 14.

In a ventilator gas pump, piston 14 may be of graphite material while cylinder 15 is of glass such as Pyrex® or the like. In one embodiment, the liner sleeve or dampening device 12 may be of any suitable acoustic dampening material having a low acoustic frequency so as to absorb harmonic vibrations resulting from a graphite piston sliding in a glass cylinder, although the piston and cylinder may be of different materials in alternative embodiments. The acoustic frequency of the material of liner sleeve 12 is less than that of the piston material. Plastic material is suitable for manufacture of the acoustic dampening sleeve due to its low acoustic frequency, easy fabrication, and low cost. However, other low acoustic frequency materials such as rubber may be used in alternative embodiments. In one embodiment, a relatively hard plastic material such as acrylic plastic or the like may be used for the liner sleeve. In one embodiment, the liner sleeve 12 was of VisiJet® 200 plastic, manufactured by 3D Systems of Rock Hill, S.C.

In the embodiment of FIG. 1, liner sleeve 12 is a straight cylindrical tube having an outer diameter which is approximately the same as the inner diameter of piston 14, so that it is a close fit inside the piston. The liner sleeve may be mechanically attached inside the piston in any suitable manner, for example using adhesive, or may be simply press fitted into the piston with no additional attachment means. The sleeve 12 is designed to have a close tolerance or friction fit to the piston so that there is wall to wall or face to face contact between sleeve 12 and piston 14 along the entire length of the sleeve.

Although liner sleeve 12 fits inside the piston in the embodiment of FIG. 1, it may alternatively be designed to fit around the outside of the piston 14, as illustrated in FIG. 2, with the inner surface 12 in face to face contact with the outer surface of piston 14.

The liner sleeve 12 has a lower acoustic frequency than the graphite material of piston 14 and tends to absorb harmonic vibration in the graphite piston/glass sleeve assembly. The lower acoustic frequency of the sleeve coupled to the piston wall shifts the natural harmonic frequency of the piston to a lower frequency, so that the piston is unable or less likely to sustain a harmonic vibration. This reduces or eliminates the squeaking noise which would otherwise occur when the graphite piston alone slides in the cylinder. The dampening action can be compared to what happens when a tin can is struck with a hard object so that it “rings”, but if a thin rubber sheet is glued to the inside of the can, it no longer rings when struck, but instead makes more of a dull, “thunk”-like noise.

In one embodiment, liner sleeve 12 is a straight tubular member with smooth cylindrical inner and outer walls, as illustrated in FIGS. 1 and 2. FIGS. 3 and 4 illustrate an alternative embodiment in which a liner sleeve or acoustic dampening device 25 is designed with an annular rib 26 extending around its outer surface for mating engagement in a corresponding annular groove 28 in the inner surface of the piston wall (see FIG. 4). In an alternative arrangement, the outer surface of sleeve 25 may have an annular groove for engagement with a corresponding annular rib on the inner surface of the piston wall. Alternatively, a sleeve for fitting over the outer surface of the piston as in FIG. 2 may have an annular rib or groove on its inner surface for engagement with a corresponding groove or rib, respectively, on the outer surface of the piston. In alternative embodiments, other mateable formations may be provided on the opposing surfaces of the liner sleeve and piston to aid in attachment, such as holes, slots, ribs, bumps, grooves, flanges, and the like.

An angled slot 28 is cut through the wall of sleeve 25 so as to extend at an angle to the central axis of the sleeve. This allows the sleeve to be compressed during installation into the piston. The slot 28 may be parallel with the sleeve axis in alternative embodiments, but the angled slot allows for additional compression as the opposing angled faces 29 of the slot slide against one another. In one embodiment, slot 28 may extend at an angle of around 45 degrees to the central longitudinal axis of the sleeve. The angled slot can be of minimal width and helps to ensure that the sleeve is not displaced during operation. Since the slot allows for compression of the sleeve during installation, the sleeve may have an oversized outer diameter slightly greater than the inner diameter of the piston in this embodiment, to ensure full wall contact between the liner sleeve and piston faces. The slot removes excessive stress or the need for expensive tolerances during manufacture of the part, due to the compressibility of the sleeve.

Although the sleeve is generally tubular or cylindrical in the above embodiments, it may be of a variety of different configurations or shapes as long as it has a face substantially matching the opposing piston face and in close contact with that face along the entire length of the sleeve or dampening device. Although a single, one-piece sleeve is used as the dampening device in the foregoing embodiments, two or more sleeves may be inserted into the piston in alternative embodiments, providing a multi-layer acoustic dampening device. The sleeves may be of the same materials or different materials. In one embodiment, a first sleeve may be of hard plastic while a second sleeve is of rubber. Multiple dampening sleeves of different materials may help to tune the piston at a desired acoustic frequency or may aid in attachment to the piston due to the tendency of rubber to adhere to the harder plastic material.

In the embodiments of FIGS. 1 to 4, the wall thickness of liner sleeve 25 is about the same as the wall thickness of piston 12, but a thicker or thinner liner sleeve may be provided in alternative embodiments, depending on the noise dampening qualities required for a particular application. As noted above, the liner sleeve 12 or 25 may be used for reducing squeaking noise during operation of a gas pump piston assembly, such as the piston assembly of a medical ventilator. Such squeaking noises can be annoying or disruptive during use of medical ventilators in a homecare environment, hospital, or during transportation of patients. For example, the HT70 ventilator of Newport Medical Instruments, Newport Beach, Calif. has a pump piston assembly which includes two or four piston assemblies driven by a common pump drive unit, as described in more detail in U.S. Pat. No. 7,654,802 referenced above. Squeaking noises as described above have been encountered during use of this pump, and can be reduced or avoided by installation of a noise dampening device or liner sleeve as described above in connection with FIGS. 1 to 4 on each of the pistons of the pump.

FIG. 5 illustrates part of a dual piston ventilator pump 40 which has two pistons 14 driven by a reciprocating drive assembly 30 identical to that described in U.S. Pat. No. 7,654,802 referenced above, which has a rotational drive or motor 32 and a sliding drive linkage 34 at each end of the rotational drive which converts rotation of the opposing drive or crank shafts 35 into linear reciprocating movement of the pistons. The glass cylinder and pump housing in which each piston slides is omitted in FIG. 5 for clarity. Pistons 14 of FIG. 5 are identical to the piston 14 described above in connection with FIGS. 1 to 4. A liner sleeve 25 is press fitted inside the inner diameter of each piston 14 for acoustic dampening purposes. Alternatively, a straight tubular liner sleeve 12 as illustrated in FIG. 1 may be fitted inside pistons 14, or the sleeve may be mounted around the outer surface of the piston, as in FIG. 2. In a four piston pump assembly, a similar acoustic dampening sleeve or liner sleeve is associated with all four pistons to reduce or eliminate squeaking noises.

In one embodiment of a liner sleeve designed for installation in the inner diameter of a pump of the HT70 ventilator described above, the sleeve has an inner diameter of about 0.98 inches, an outer diameter of approximately 1.14 inches, and a height of around 0.5 inches. However, sleeves may be made of any suitable dimensions depending on the dimensions of the piston in which they are to be fitted. The outer diameter should be as close as possible to the inner diameter of the piston, or slightly greater than the inner diameter of the piston in the case of a sleeve 25 with an angled slot 26 as in FIG. 4. The goal is to have an interference or friction fit between the opposing surfaces of the piston and liner sleeve, so as to shift or change the natural frequency of the graphite material of the piston to the combined frequency of the graphite material and the plastic material of the liner sleeve. This helps to avoid or limit acoustic resonance or squeaking noises. With a graphite material piston alone, acoustic resonance is found to occur in the frequency range correlated to 200 to 350 rpm. The acoustic resonance frequency of the combined plastic and graphite material in the assemblies of FIGS. 1, 2, 4 and 5 is shifted away from this range so that little or no sound due to resonance is emitted in the normal operating range of the pump. A number of HT70 ventilator pumps known to have a squeaking problem were modified to include a liner sleeve as described above, and the pumps were found to have no squeaking noise in a five day burn-in test. In other embodiments, a liner sleeve 12 or 25 may alternatively be installed on the pistons of other types of pumps or other devices in which piston noise or squeaking is a problem.

The above description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles described herein can be applied to other embodiments without departing from the spirit or scope of the invention. Thus, it is to be understood that the description and drawings presented herein represent a presently preferred embodiment of the invention and are therefore representative of the subject matter which is broadly contemplated by the present invention. It is further understood that the scope of the present invention fully encompasses other embodiments that may become obvious to those skilled in the art and that the scope of the present invention is accordingly limited by nothing other than the appended claims.

We claim: 

1. A piston assembly, comprising: a cylinder of a first material; at least one piston of a second material different from the first material slidably mounted for reciprocating motion in the cylinder, the piston being hollow and having an inner surface of a first diameter and an outer cylindrical surface of a second diameter; at least one liner sleeve mounted on one of said inner and outer surfaces of the piston and having a close tolerance fit to the diameter of the piston surface on which it is mounted, whereby the opposing surfaces of the liner sleeve and piston are in face to face engagement; and the liner sleeve being of an acoustic dampening material which has a lower acoustic frequency than the piston material.
 2. The piston assembly of claim 1, wherein the liner sleeve is mounted on the inner surface of the piston.
 3. The piston assembly of claim 2, wherein the liner sleeve has a central longitudinal axis and opposite axial ends, and the sleeve has a slot extending between the axial ends configured to allow compression of the sleeve during installation into the piston.
 4. The piston assembly of claim 3, wherein the slot extends at an angle to the central longitudinal axis of the sleeve.
 5. The piston assembly of claim 1, wherein the liner sleeve is mounted on the outer surface of the piston between the piston and cylinder.
 6. The piston assembly of claim 1, wherein the liner sleeve is a straight cylindrical tube.
 7. The piston assembly of claim 1, wherein the surface of the liner sleeve facing the piston surface has a first mating formation and the opposing surface of the piston has a second mating formation configured for alignment and engagement with the first mating formation when the sleeve is installed.
 8. The piston assembly of claim 7, wherein one of said mating formations comprises an annular rib and the other mating formation comprises an annular groove.
 9. The piston assembly of claim 1, wherein the acoustic dampening material is plastic or rubber.
 10. The piston assembly of claim 9, wherein the acoustic dampening material of the sleeve is an acrylic plastic material.
 11. The piston assembly of claim 1, wherein the cylinder is of glass material and the piston is of graphite material.
 12. A gas pump assembly for a medical ventilator, comprising: at least a first gas piston assembly comprising a cylinder having a longitudinal axis and a first hollow piston slidably mounted for reciprocating movement in the cylinder and having a piston rod extending out of one end of the cylinder; a drive unit drivably linked to the piston rod and configured for driving the piston back and forth in the cylinder; the piston being hollow and having an inner surface of a first diameter and an outer cylindrical surface of a second diameter; and at least one liner sleeve mounted on one of said inner and outer surfaces of the piston and having a close tolerance fit to the diameter of the piston surface on which it is mounted, the liner sleeve being of an acoustic dampening material which has a lower acoustic frequency than the piston material.
 13. The assembly of claim 12, further comprising a second piston assembly identical to the first piston assembly and having a second cylinder with a longitudinal axis extending parallel to the longitudinal axis of the cylinder of the first piston assembly, a second hollow piston slidably mounted for reciprocating movement in the second cylinder and having a second piston rod extending out of the one end of the second cylinder, a second linkage connecting the drive output of the drive motor to the second piston rod, and a second liner sleeve identical to said one liner sleeve and mounted on one of the inner and outer surfaces of the second piston.
 14. The assembly of claim 12, further comprising a plurality of additional piston assemblies identical to said first piston assembly, each additional piston assembly having a piston rod drivably linked to said drive motor and a respective hollow piston slidably mounted in the cylinder of said additional piston assembly, and each hollow piston having a liner sleeve of acoustic dampening material mounted on one of the inner and outer surfaces of the respective hollow piston. 